The invention relates to a motor controller and more particularly to a motor controller for driving a fluid impeller and still more particularly to a motor controller for driving a fluid impeller to provide a specific fluid flow rate.
It is known to employ electric motors to drive fluid impellers such as fan blades or blower cages in air moving apparatus. Such apparatus are typically used in heating, ventilation and air conditioning applications.
It is further known that heating, ventilation and air conditioning systems require a, constant fluid flow in order to operate efficiently. Fluid resistance in the ducting of such systems typically varies with time as a result of various in fluid paths and duct openings, For example, every adjustment of a ventilation opening causes a fluid resistance change in the ducting.
It is known that blower torque must be adjusted to compensate for variable fluid resistance if constant fluid flow is to be maintained.
Various methods and apparatus are known to adjust blower torque in response to variations in fluid resistance or load. Typically, fluid flow may be measured directly by fluid flow transducers which are immersed in the fluid flow path. An electrical signal is typically fed back from the transducers to a microprocessor system or an electric circuit which is designed to adjust the speed of a blower motor to approach a predetermined constant value. Such systems are often too expensive or comprise components that are too large for use in practical heating, ventilation and air conditioning applications.
It is known that the magnitudes of a phase current in a blower motor drive circuit is related to the magnitude of fluid flow which is impelled by the blower. It is further known to provide a constant fluid flow by comparing a measured phase current of a blower motor drive circuit with an empirically determined ideal reference phase current for a specific constant fluid flow to determine an error phase current signal. The empirically determined reference phase current value is typically stored in a look-up table in the memory of a microprocessor system. It is further known to manipulate an error phase current signal so that it is suitable for input as an index to a pulse width modulator in a motor control circuit wherein the motor control circuit is caused to change motor speed to reduce the error phase current signal. The error phase current signal is reduced as the measured motor current approaches the ideal constant flow reference phase current.
Such methods may provide imprecise flow control because phase currents are known to fluctuate and are typically noisy. Furthermore such methods require added cost because they require current measurement feedback loops.
It is desirable to provide a constant fluid flow motor controller of reduced complexity by means not requiring direct measurement of fluid flow rate or motor current nor requiring any dedicated feedback sensor components.
Accordingly, the invention provides a specific fluid flow motor controller by employing a theoretically derived algorithm to operate on critical motor parameters internal to a variable frequency drive.
The invention determines a speed at which a motor 14 is operating pursuant to at least a second order polynomial equation, as follows:
N=C2RI2+C1RI+C0Rxe2x80x83xe2x80x83(1)
wherein N is the operating speed of the induction motor at the direct current bus current I, while operating at a specific voltage and frequency and C2R, C1R and C0R are motor constants of proportionality for the required voltage-frequency index R.
Equation (1) characterizes the steady-state control relationship between the operating speed N and the direct current bus current I. The set of constants of proportionality C2R, C1R and C0R are specific motor constants of proportionality for the voltage-frequency index R. The size of the constant for varying R set is chosen appropriately to meet the required fineness of control.
The algorithm of the invention further another at least second-order polynomial equation, as for describing blower torque, as follows:
Tb=A2FNb2+A1FNb+A0Fxe2x80x83xe2x80x83(2)
wherein Tb is the torque required by the blower at speed Nb to deliver a specific flow rate and A2F, A1F and A0F are specific blower constants of proportionality for the required flow rate F.
Equation (2) characterizes the steady-state control relationship between the blowers speed Nb and the required blower torque to deliver the desired rate of fluid flow. The set of constants of proportionality A2F, A1F and A0F are deduced uniquely for each blower design. The size of the constant set for varying F is chosen appropriately to meet the require range of flow control.
The algorithm of the invention employs a further at least second-order polynomial equation, as for describing motor torque, as follows:
Tm=B2RNm2+B1RNm+B0Rxe2x80x83xe2x80x83(3)
wherein Tm is the torque produced by the induction motor at a speed Nm while operating with a specific voltage-frequency index R and B2R, B1R and B0R are specific motor constants of proportionality for the voltage-frequency index R.
Equation (3) characterizes the steady control relationship between an induction motor speed Nm and the developed motor torque for the operating voltage-frequency index. The set of constants or proportionality B2R, B1R and B0R are deduced uniquely for each induction motor and drive control electronics design to be used. The size of the constant set for varying R is chosen to meet the required fineness of control.
The invention employs a microprocessor system to implement a steady state control algorithm and a transient control algorithm. The transient control algorithm comprises a start-up procedure which controls the motor/blower system until it approaches a steady state condition. Under steady state conditions, Tb=Tm if Nb=Nm=N as when the motor is directly attached to the blower. Otherwise, the product of torque and speed for the motor is equals the product of torque and speed for the blower.
When the control system is started it executes the transient control algorithm for a start-up period. During the start-up period the microprocessor system changes the voltage-frequency index of the controller to cause the motor speed to ramp up from rest or zero rotations per minute to a desired steady state speed. The microprocessor system computes the speed value numerically by manipulating a measurement of the direct current bus current in the induction motor/blower system according to equation (1). After the start-up period, the microprocessor system executes the steady state control algorithm. The start-up period is chosen based upon the rotational inertia of the particular motor/blower system so that the speed will reach the desired steady state value before the end of the start up period.
While executing the steady state control algorithm, the microprocessor system calculates the required blower torque Tb using equation (2). The microprocessor system reads a user input, typically a selector switch bank, which provides a desired fluid flow rate signal (ie., an input value for F) and selects the matching constants A2F, A1F and A0F from memory. The microprocessor system computes the motor speed by manipulating a measurement of the direct current bus current according to equation (1). The microprocessor system calculates the required blower torque using equation (2) to operate on the selected flow constants and actual motor speed.
While executing the steady state control algorithm, the microprocessor system also calculates the developed motor torque Tm by using equation (3). The motor speed is determined by manipulating a measurement of the direct current bus current according to equation (1) and the motor constants B2R, B1R and B0R are read from memory as a function of the operating voltage-frequency index.
While executing the steady state algorithm the microprocessor system compares the computed values of Tb and Tm and adjusts the voltage-frequency index to force Tb and Tm to converge. If Tb=Tm the microprocessor system makes no changes to the voltage-frequency index. If Tb greater than Tm the microprocessor system modifies the voltage-frequency index to cause Tm to increase. If Tb less than Tm the microprocessor system modifies the voltage-frequency index to cause Tm to decrease. The microprocessor system waits for a settle time after each modification of the voltage-frequency index wherein the settle time is determined by the motor/blower system rotational inertia. The microprocessor system continuously repeats the steps of the steady state algorithm.
Again, the foregoing assumes that Nb=Nm=N. Also, the foregoing assumes that the main supply voltage is constant. However, the algorithm may optionally apply corrections for either, including for supply voltage variation where improved flow control resolution is required.
It is an advantage of the invention to provide a specific fluid flow rate without the need for any motor speed components and without relying on motor phase current measurements.